Starch has sometimes been suggested for use in elastomer compositions for various purposes, including tires, particularly as a starch/plasticizer composite. For example, see U.S. Pat. No. 5,672,639.
Such starch composites may be used in combination with various other fillers, particularly reinforcing fillers for elastomers such as, for example, carbon black, silica, vulcanized rubber particles, short polymeric fibers, kaolin clay, mica, talc, titanium dioxide and limestone. Carbon black and/or silica, particularly precipitated silica, may be preferred. Such short fibers may be, for example, fibers of cellulose, aramid, nylon, aramid, polyester and carbon composition.
U.S. Pat. Nos., for example, 5,403,923; 5,374,671; 5,258,430 and 4,900,361 disclose a preparation and use of various starch materials.
As pointed in the aforesaid U.S. Pat. No. 5,672,639, starch is typically represented as a carbohydrate polymer having repeating units of amylose (anydroglucopyranose units joined by glucosidic bonds) and amylopetin, a branched chain structure, as is well known to those having skill in such art. Typically, starch may be composed of about 25 percent amylose and about 75 percent amylopectin. (The Condensed Chemical Dictionary, Ninth Edition (1977)), revised by G. G. Hawley, published by Van Nostrand Reinhold Company, page 813). Starch can be, reportedly, a reserve polysaccharide in plants such as, for example, corn, potatoes, rice and wheat as typical commercial sources.
While starch may have been previously suggested for use in rubber products, starch by itself, typically having a softening point of about 200.degree. C. or above, is considered herein to have a somewhat limited use in many rubber products, primarily because rubber compositions are normally processed by preliminarily blending rubber with various ingredients at temperatures in a range of about 140.degree. C. to about 170.degree. C., usually at least about 160.degree. C., and sometimes up to 180.degree. C. which is not a high enough temperature to cause the starch (with softening temperature of at least about 200.degree. C.) to effectively melt and efficiently blend with the rubber composition. As a result, the starch particles tend to remain in individual domains, or granules, within the rubber composition rather than as a more homogeneous blend.
Thus, it is considered herein that such softening point disadvantage has rather severely limited the use of starch as a filler, particularly as a reinforcing filler, for many rubber products.
It is considered herein that use of a starch/plasticizer composite, or composition, with a softening point significantly lower than that of the starch alone may allow the starch to be more easily mixed and processed in conventional elastomer processing equipment. Such composites, as pointed in the aforesaid U.S. Pat. No. 5,672,639, may be, for example, a composite of starch and plasticizer.
A silica coupler may be used in conjunction with such starch composite and with silica, such as precipitated silica, to enhance the reinforceability, as pointed out in U.S. Pat. No. 5,672,639 which has a moiety reactive with the surface of the silica (i.e.: silicon hydroxide) and the surface of the starch composite and another moiety interactive with a sulfur-curable elastomer.
Coupling agents such as, for example, an organosilane polysulfide having an average of from 3.5 to 4 sulfur atoms in its polysulfidic bridge has been used for coupling precipitated silica to elastomers.
Exemplary of such organosilane polysulfide is bis-3(triethoxysilylpropyl)polysulfide with an average of about 3.8 sulfur atoms in its polysulfidic bridge. It is envisioned that such polysulfide can be a sulfur donor, by liberating free sulfur, during typical high shear mixing of a rubber composition at an elevated temperature such as, for example, at temperatures of 100.degree. C. and above, depending somewhat upon the polysulfide used and the mixing temperature and time.
The small amount of free, liberated, sulfur is then available to combine with and/or possibly partially vulcanize, a diene-based elastomer.
It is, however, considered herein that an organosilane polysulfide blend, which is primarily a disulfide having an average of about 2.6 or less sulfur atoms in its polysulfidic bridge, is not normally a good sulfur donor under such mixing conditions, due to the relatively strong sulfur-to-sulfur bonds typical of an organosilane disulfide--as compared to an organosilane polysulfide with an average of at least 3.5 sulfur atoms in its polysulfidic bridge.
Accordingly, it is considered herein that, for an organosilane polysulfide which contains an average of less than 2.8 and particularly within a range of about 2 to about 2.6, sulfur atoms in its polysulfidic bridge, the liberation of free sulfur, if any, occurs at a relatively slow rate during a high shear rubber mixing stage, even at a mixing temperature in a range of about 150.degree. C. to about 185.degree. C. depending somewhat upon the overall mixing conditions, including the mixing time itself.
Bis-(3-triethoxysilylpropyl)disulfide, as a variety of organosilane disulfide, is also taught as being useful in silica-containing sulfur-vulcanizable elastomer compositions, even as a high purity form of such disulfide in, for example, U.S. Pat. No. 4,046,550 and German Patent Publication DT 2,360,471. However, it is considered herein that such disulfide does not ordinarily readily liberate free sulfur in such aforementioned rubber/silica/coupler mixing operation.
For examples of organosilane polysulfides for use as silica couplers, see U.S. Pat. Nos. 4,076,550; 4,704,414; and 3,873,489.
For examples of organosilane disulfides added in a preparatory, non-productive, rubber composition mixing stage, along with a small amount of free sulfur, see U.S. Pat. Nos. 4,076,550; 5,580,919 and 5,674,932.
In practice, sulfur-vulcanized elastomer products are typically prepared by thermomechanically mixing rubber and various ingredients in a sequentially step-wise manner followed by shaping and curing the compounded rubber to form a vulcanized product.
First, for the aforesaid mixing of the rubber and various ingredients, typically exclusive of free sulfur and sulfur vulcanization accelerators, the elastomer(s) and various rubber compounding ingredients are typically blended in at least one, and usually at least two, sequential, preparatory thermomechanical mixing stage(s) in suitable mixers, usually internal rubber mixers. Such preparatory mixing is often referred to as "non-productive mixing", or "non-productive mixing steps or stages". Such preparatory mixing is usually conducted at temperatures in a range of about 140.degree. C. to 190.degree. C. and more often in a range of about 140.degree. C. or 150.degree. C. to about 185.degree. C.
Subsequent to such sequential, preparatory mix stage(s), free sulfur and sulfur vulcanization accelerators, and possibly one or more additional ingredients, are mixed with the rubber compound, or composition, in a final, productive, mix stage, typically at a temperature within a range of about 100.degree. C. to about 130.degree. C., which is a lower temperature than the temperatures utilized in the aforesaid preparatory mix stage(s) in order to prevent or retard premature curing of the sulfur-curable rubber, which is sometimes referred to as "scorching", of the rubber composition.
Such sequential, non-productive, mixing steps, and the subsequent productive mixing step are well known to those in the rubber mixing art.
By thermomechanical mixing, it is meant that the rubber compound, or composition of rubber and rubber compounding ingredients, is mixed in a rubber mixture under high shear conditions where it autogeneously heats up, with an accompanying temperature rise, as a result of the mixing primarily due to shear and associated friction within the rubber mixture in the rubber mixer.
Such thermomechanical rubber compound mixing procedure and associated shear and accompanying temperature rise aspects are well known to those having experience in the rubber preparation and mixing art.
In practice, it is believed that the inventors' prescribed procedure of (1) adding an organosilane disulfide compound in a non-productive rubber composition mixing stage, or step, followed by (2) subsequently adding an organosilane polysulfide compound with a average of from 3.5 to 4.5 sulfur atoms in its polysulfidic bridge together with a small amount of free sulfur in a productive rubber composition mixing stage for a combination of starch composite and silica-based reinforced rubber composition, particularly as a means of controlling the associated sulfur/elastomer interaction as well as interaction with a silane/starch, as well as additional silane/filler (ie: silane/silica) composite network product created by the reaction of the organosilane component of the organosilane disulfide compound with the starch composite reinforcement and with the silica-based reinforcement in the prior, preparatory, mixing stage(s), is novel and inventive in view of past practice.
In one aspect, it is believed that a decoupling of an initial silane/starch composite and silane/silica reaction (via the organosilane component of the organosilane disulfide compound) and a subsequent release of free sulfur, and additional silane reaction, (via the subsequent addition of the organosilane polysulfide compound) to interact with the elastomer(s) in a sequential rubber composition mixing procedure is accomplished by using a combination of separate and selective addition of an organosilane disulfide compound (I) and subsequent addition of an organosilane polysulfide compound (II) followed by vulcanizing the rubber composition according to the procedure of this invention is a significant departure from past practice.
In the description of this invention, the organosilane disulfide compound is used to describe an organosilane polysulfide compound having an average of from 2 to about 2.6 sulfur atoms in its polysulfidic bridge and the organosilane polysulfide compound is used to describe an organosilane polysulfide compound having an average of from about 3.5 to about 4.5 sulfur atoms in its polysulfidic bridge.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber, or elastomer".
In the description of this invention, the terms "rubber" and "elastomer" if used herein, may be used interchangeably, unless otherwise prescribed. The terms such as "rubber composition", "compounded rubber" and "rubber compound", if used herein, are used interchangeably to refer to "rubber which has been blended or mixed with various ingredients and materials" and "rubber compounding" or "compounding" may be used to refer to the "mixing of such materials". Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
A reference to an elastomer's "Tg", if used herein, refers to a "glass transition temperature" which can be determined by a differential scanning calorimeter at a heating rate of 10.degree. C. per minute.